Glycaemic index (GI)
The glycaemic index (GI) ranks carbohydrate foods by how quickly they raise your blood sugar compared with pure glucose, on a scale of 0 to 100.
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How GI works
Researchers feed people a portion of a food containing 50 g of carbohydrate and measure their blood sugar over the next two hours, then compare it with 50 g of glucose (GI 100).
Fibre, fat, protein, acidity, how ripe a food is, how it's cooked and how much it's processed all change its GI. That's why the same food can have different values in different studies.
GI of common foods
| Food | GI | Band |
|---|---|---|
| White bread | 75 | High |
| Cornflakes | 81 | High |
| Potato, boiled | 78 | High |
| White rice, boiled | 73 | High |
| Rolled oats porridge | 55 | Low |
| Sweet potato, boiled | 63 | Medium |
| Brown rice, boiled | 68 | Medium |
| Spaghetti, white, boiled | 49 | Low |
| Banana | 51 | Low |
| Apple | 36 | Low |
| Chickpeas | 28 | Low |
| Lentils | 32 | Low |
| Milk, full fat | 39 | Low |
Values are averages from many studies; individual products and brands can differ2. Some nutritious foods, like boiled potatoes, are high GI, while some less healthy ones, like chocolate and crisps, are lower because of their fat. GI alone doesn't tell you whether a food is healthy.
Glycaemic load: the more useful number
GI ignores portion size. Glycaemic load (GL) fixes that: GL = GI × grams of carbohydrate in your portion ÷ 100.
| GI | Carbs in a portion | Glycaemic load | |
|---|---|---|---|
| Watermelon, 1 slice (120 g) | 76 | about 9 g | about 7 (low) |
| White rice, 150 g cooked | 73 | about 42 g | about 31 (high) |
Watermelon has a high GI but so little carbohydrate per slice that its real effect is small.
Does GI matter for lifters?
- Day to day: lower-GI, high-fibre carbs (oats, wholegrains, beans) keep you fuller and give steadier energy.
- Around training: higher-GI carbs are useful when you need fuel fast, like a banana before a session, or refuelling if you train twice in one day3.
- For health: fibre and wholegrains matter more than GI numbers. Higher-fibre diets are linked with lower risk of heart disease, type 2 diabetes and bowel cancer4.
Common myths
MythHigh-GI foods make you fat.
FactWeight change comes down to total calories. GI can affect how full you feel, but a high-GI food eaten within your calories won't cause fat gain by itself.
MythLow GI means healthy.
FactIce cream and chocolate have a low GI because of their fat. Look at the whole food, not just the number.
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References
- Atkinson FS, Foster-Powell K, Brand-Miller JC. International tables of glycemic index and glycemic load values: 2008. Diabetes Care. 2008;31(12):2281-2283. doi:10.2337/dc08-1239
- Atkinson FS, Brand-Miller JC, Foster-Powell K, Buyken AE, Goletzke J. International tables of glycemic index and glycemic load values 2021: a systematic review. Am J Clin Nutr. 2021;114(5):1625-1632. doi:10.1093/ajcn/nqab233
- Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29 Suppl 1:S17-S27. doi:10.1080/02640414.2011.585473
- Reynolds A, Mann J, Cummings J, Winter N, Mete E, Te Morenga L. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393(10170):434-445. doi:10.1016/S0140-6736(18)31809-9